Thin metal films and method of making same



April 28, 1959 A. z. czlPoTT ETAL 2,884,508

THIN METAL FILMS AND METHOD OF MAKING SAME Filed OCT.. l, 1956 cfy F2 aya,

' IN VEN TORS.

Ffef. BY v 46E/v7' United States Patent() THIN METAL FILMS AND NIETHOD OF MAKING SAME Akos Z. Czipott, Alhambra, and Acey L. Floyd, Duarte, Calif., assignors, by mesne assignments, to Dresser Industries, Inc., Dallas, Tex., a corporation of Delaware Application October 1, 1956, Serial No. 613,172

7 Claims. (Cl. 201-63) This invention relates to photoconductive cells, and more particularly to a photoconductive cell having a layer or coating of a photosensitive substance, and electrical conductor means, such as electrodes or terminals, making ohmic contact with the photosensitive material. The term ohmic contact as herein used means an electrical contact or boundary having substantially the same resistance to current flow in either direction.

It has heretofore been known to produce photosensitive cells using a layer or lm of cadmium sulphide deposited from sublimated powder as the photosensitive element. In such cells the cadmium sulphide is deposited as a layer between and in contact with conductor means, such as thin conductive films of chromium, which serve as electrodes. A procedure illustrative of this mode of cell construction is set forth in copending patent application Serial No. 489,397, filed February 21, 1955, for example. It has been found that the electrical contact between the conductive films of chromium and the photoconductor layer are not always ohmic, and that the difference in resistance to current flow in opposite directions may be of the order of 103, due, apparently, to the rectifying nature of the metal-to-semiconductor contact. The lack of uniformity of the contact resistivity from cell to cell is an undesirable characteristic attending the previously employed thin conductive lms or electrodes.

Since in a photoconductive cell of the powder layer or film type, current must liow from a conductor into a 'thin layer or film of powder or polycrystalline semiconductor and rfrom the latter into a second'conductor, the importance of ohmic contact between the conductors and semiconductor film is apparent. Without substantially low resistance ohmic contact, cell `sensitivity of only a low order is obtainable, since current can only with difficulty be forced across one of the zones of contact. Further, with non-ohmic Contact between conductor and Senliconductor, the voltage versus current characteristic of such a cell is non-linear. Linearity of this characteristic would be highly desirable and valuable.

It has been determined that by employing as a conductor in the contact zone, indium, or gallium, or any 2,884,51-)8N Patented Apr. 28, 1959 a base and the films are to be separated by only small distances, as in reticle-type cells, the metal is deposited as a continuous layer over the base and over thin lines of hardened photoengravers glue. When the glue is subsequently removed the overlying chromium or other hard metal falls away leaving a metal-free area in the form of a fine line-like area on the base. If, however, a substantial layer of chromium or other hard metal is deposited, the metal layer has sufficient strength to maintain itself intact in parts after removal of the underlying glue, and thus forms an effective electrical connection between the conductive films on the base. Since the films are required to be electrically isolated, a cell having such bridges or shorts is of little or no value. Another advantage of gallium and indium as a conductor material in such cells is that these conductors can endure without deterioration much higher temperatures than, for example, chromium. Cells of the type under consideration are during manufacture subjected to high baking temperatures.

Stages in the production of an exemplary cell in accord with the concept of the invention are illustrated in the drawings, in which:

Figure 1 is a pictorial representation of the exemplary reticle-type photoconductive cell, with certain areal dimensions exaggerated in the interest of clear representation;

Figures 2 through 5 are fragmentary views illustrating a photoconductive cell at certain different stages of its construction, and with certain dimensions exaggerated;

Figures 6, 7, and 8 are fragmentary views on a larger scale illustrating features of cell construction at several stages of manufacture, the dimensional scale being exaggerated to facilitate clear illustration.

Referring to the drawings, Figure l shows somewhat pictorially one exemplary photoconductive cell of the reticle type constructed in accord with the principles of the present invention. Thcrein a suitable base 10 of electrically non-conductive material, such as glass or quartz, carries on a surface thereof an arrangement of discrete electrically-conductive thin films 12 of indium, gallium,

p or other metal or alloy having a low electronic work function. Terminals such as 13 and 14 are applied to selected individual lms 12.; and the terminals may similarly be of indium or the like, and applied in any suitable manner or as portions of the appropriate discrete films 12. The discrete films 12 are separated each from adalloy of the two, or other metal having a low electronic f jacent films by narrow line-like areas 15 which are free of metal. Deposited over at least the line-like areas 15 and contacting the edges of the metallic films, is a coating or layer of photoconductive material, such as cadmium sulphide, which may also overlie and completely cover the films 12. The coating or layer of photosensitive material is very thin, and is indicated in Figure 1 by the scattered dots on the metal films and upper surface of base 10. The photoconductive layer need not cover the entire surface of the base, but should be substantially continuous over at least the line-like areas 15 between the metal films. The metal films 12 may be applied to the base according to any desired procedure, but preferably are applied following a procedure such as is outllined in the aforementioned application Serial No.

example, is employed in forming the conductive films on i 489,397, modified in respects hereinafter set out in detail.

Successive stages in the production of a finished reticle type cell are illustrated in Figures 2 through 5, in which the relative dimensions of the films or coatings are exaggerated to facilitate illustration. In Figure 2, a fragment of a non-conductor base 10a, of glass for example, has

applied thereto a coating 20 of photoengravers glue. In Figure 34', the same base'la is illustrated as it appears following exposure of the glue layer to illumination masked by a photonegative, and development. At this stage, areas 21 of the surface of the base 10a have been uncovered by removal of glue, leaving a network of connected lines 22 of glue, and a marginal portion 22 of the glue layer, intact. A iilm yor layer of indium, gallium, or alloy thereof, is deposited over the remaining glue and ther'exposeda'reas 2liv of the surface of the hase. This is accomplished, for example, by vacuum sublimation of the metaljduringwhich operation the sublimation product is deposited on the' base surface. Thus there is deposited on' the cell surface and over the network of glue a substantially continuous electrically-conductive sheet or lm 23 ofthe metal (see Figure 4). The film 23 bridges the lines 22 of glue in a fashion illustrated on exaggerated scale in the fragmentary view in Figure 6.

'Following deposition of lm 23 of the sublimed metal, the remaining glue is carefully removed by soaking the entire cell structure in NaOH solution for approximately thirty minutes, without rubbing or other abrasion. The glue softens and dissolves from beneath the overlying porousiilm of metal 23; and that part of the metal film thus left unsupported by removal of the glue breaks into small fragments which are washed away in a washing operation which preferably includes a sodium hypochlorite rinse followed by a distilled water rinse. Removal of thev glue and those portions of the metallic lm which werev supported by the glue leaves discrete metal lms 24 adhering to vthe base 10a, as indicated in Figure ;"the filmsbeing surrounded and separated by metal-free surface areas ofthe base. The distances of separation of the discrete Iilms or areas of metal are governed by the widths of the opaque lines or areas in the photonegative used'in exposing the glue layer, and-may besuch as is desired." The separation of any two such metallic ilms is preferably uniform and of a very smalldimension. For "example, the line-like metal-free areas separating next-adjacent vindiumy films `may be of the order of .003 inch! wide.Vr A section, to an exaggerated dimensional scale, through two of the indium films 24 and interveningiimetal-freeV space is illustrated in Figure 7, the linelike area separating the two lms being therein indicated by numeral 25 'and the previously removed bridge of indium or metal overlying area 25 being indicated Aby dotted ilines. i l

Following vthe steps hereinbefore outlined, the cell with its separatemetal films is preferably suitably heated to renderl the'metal more rmly adherent to the glass. This heatmtreaunent may consist, for example, in heating lthe cellfatvabout 400? C. for approximately thirty minutes. If during Athe, heat treatment an oxide layer isformed, it maybe later removed byV lightly Washing the lm with dilutewHCvl acid. While the heat treatment may under somelcircumstances kbe omitted, preferably it is included forftili reason and purpose stated. An additional advantage of employing indium'rather ythan chromium or other hard metal is thatthe former is capable of` withstanding longer heating athigh temperatures than is a hard metal.

The ,cell at ,this stage of manufacture is ready for application of thepotentially or actively `photoconductive material,l which may be applied in accord. with the procedure outlined in the aforementioned copending` application. The photosensitive material, which may be principally cadmium sulphide, or zinc sulphideor other photoconductor, may if desired be applied to only the areas between the metal iilms 24-and so as to substantially uniformlycontact the edges of the metal films. However, as a'matter Vof convenience and in the interest of uniform contact with Ithe metal films, the photoconductive material may` be' applied as a layer or iilm overlying upper surface of the cell, as indicated by the dots in Figur'ef'l 'andas indicated in exaggeratedhform as`llayer2f6 the entire in Figure 8. The layer 26, as indicated in Figure 8, contacts the exposed edges 24a of lms 24; and, as hereinbefore indicated, provides an ohmic contact with the electrically conductive lms 24.

While a photoconductive reticle-type cell produced in the manner hereinbefore described may be used without further treatment, the cell may be subjected to other treatments, such as those indicated and described in copending patent applications hereinafter identied; and further, the cell may be given a protective coat of a translucent material such as a clear resin such as glyptol.

In certain respects this application is related to the copending patent applications of Akos Z. Czipott, Serial No. 613,177 tiled contemporaneously; and Acey L. Floyd, Serial No. 489,397, tiled February 2l, 1955, to which. applications reference is made in respect to details of procedures and materials therein disclosed.

It being evident that with the preceding disclosurel of an exemplary cell and its construction in View, modications of the procedure, materials and constructions-within the scope of the invention will occur to those skilled in the art, it is not desired to be limited to the particular exemplary details set forth, but what is claimed is:

1. A photoconductive cell comprising electrically conductive ilms with marginal portions having ohmic union with. a photoconductive material contacting such marginal portions, comprising, in combination: yan electrically non-conductive base; at least two separated discrete films offsubstantial` areal extent of metallic material selected from the class consisting Vof indium and gallium, firmly adherent to said glass base and 'each lm providing a marginal portion andeach film insulated from another lm;.fand a -layer-of photoconductive material on the cell 'basefandfat least Ibridgingl anarea between and overlappingV said marginal portions to provide a photoconductive electric current path therebetween.

2. A1photoconductiveV reticle-type cell comprising,` in combination: a glass base,` a plurality of electrically-conductivediscretelms of amaterial selected from the groupconsisting ofindium, gallium, and alloys thereof, firmly adherent on ysaid base; and arlayer composed principally of photoconductive polycrystalline cadmium sulphideoverlying an larea of said base situated between atleast two of said tilms of said material and at least a marginal. area of each of the two films adjacent said area of said base, Wherebyohmic electrical cont-act between the electrically conductive lms and the contacting layer of cadmium sulphide is effected.

3. Anohmic .photoconductive cell, comprising anfelecn'icallyinsulative hase, at least two discrete films of indiumonsaid base, and-a deposit of polycrystalline photoconductive materialon said base and electrically interconnecting said iilms of indium.

4. VAphotoconductive cell comprising, in combination: an ,insulating base member; atleast one body of a material selectedfrom the group consisting of indium, gallium andalloys thereof, firmly adhered on said base member; Iand 'ra .layer composed principally of photoconductive cadmiumsulphide overlying an area of said basemember `and situated in electrical contact with said` body, wherebyohmicelectrical contact between said layer and said body is effected.

5. An ohmic photoconductive cell comprising: an electricallyv insulative base; at least two discrete bodies composed ofra material comprisingV principally indium; and a deposit composed principally of photoconductive cadmium sulphateon said base and electrically interconnecting said bodies.

6. An ohmic photoconductive cell comprising: an electrically insulative base; at least two discrete bodies of a material comprising principally gallium; and a deposit composed, principally of photoconductive cadmium sulphate on said base and electrically interconnecting said bodies.

7. jAndohmicv photoconductive cellucomprising: an

5 electrically insulative base; `at least two discrete bodies of a material selected from the group consisting of indium, gallium and `alloys thereof; and a deposit composed principally of photoconductive -cadmium sulphate on said base `and electrically interconnecting said bodies.

References Cited in the le of this patent UNITED STATES PATENTS 2,435,889 Kerridge Feb. 10, 1948 2,482,547 Kerridge Sept. 20, 1949 2,644,852 Dunlap July 7, 1953 6 McIlvaine Oct. 6, 1953 Eisler Dec. 15, 1953 Jacobs May 22, 1956 Thomsen Oct. 2, 1956 FOREIGN PATENTS Great Britain Dec. 30, 1949 OTHER REFERENCES lo Ludwick: Indium from Steel Magazine of November 9, 1942, pages 80, 81, 122-124, Indium Digest. 

1. A PHOTOCONDUCTIVE CELL COMPRISING ELECTRICALLY CONDUCTIVE FILMS WITH MARGINAL PORTIONS HAVING OHMIC UNION WITH A PHOTOCONDUCTIVE MATERIAL CONTACTING SUCH MARGINAL PORTION, COMPRISING, IN COMBINATION: AN ELECTRICALLY NON-CONDUCTIVE BASE; AT LEAST TWO SEPARATED DISCRETE FILMS OF SUBSTANTIAL AREAL EXTENT OF METALLIC MATERIAL SELECTED FROM THE CLASS CONSISTING OF INDIUM AND GALLIUM, FIRMLY ADHERENT TO SAID GLASS BASE AND EACH FILM PROVIDING A MARGINAL PORTION AND EACH FILM INSULATED FROM ANOTHER FILM; AND A LAYER OF PHOTOCONDUCTIVE MATERIAL ON THE CELL BASE AND AT LEAST BRIDGING AN AREA BETWEEN AND OVERLAPPING SAID MARGINAL PORTIONS TO PROVIDE A PHOTOCONDUCTIVE ELECTRIC CURRENT PATH THEREBETWEEN. 